Ceiling joist question.
Here’s the situation:
I have a house that looks like a giant plus sign (+)
Each outside of the “plus ” is a gable, so in the middle I have 4 valley rafters that all come to a point in the middle.
Total height of ridge is 16′
Im installing ceiling joists at 9′.
I can nail the ceiling joists to the rafters in the front, back and both sides until I get into the middle where the valley is higher than 9′.
I know I’ll have to hang the side cj’s off the front and back cj’s(or vice versa) so my question is how do I figure the size of the cj(dbl 2x?) that I am hanging the other cj’s from?
Is there an equation or calculator somewhere online? I couldn’t find one.
Total span is about 12’6″ on all the cj’s
I’m using 2×6 for cj’s with no storage or anything above
Thanks
Edited 12/29/2008 7:40 pm ET by FCOH
Replies
I'll bump this post up and add that I would think that with the span mentioned, you are already undersized using 2x6's.
We'll have to see..................
dug fir 2x6 should be ok but we'll see
I'll probably have to beef up the joists that are being hung, seeing as how the span will be more than 13 ft
Edited 12/29/2008 9:38 pm ET by FCOH
I'll bump this up for you.
You could probably google up some span and load tables while you are waiting.
I likely have links to them.
I'll see if I can conjure up Piffin, he's good at this.
Hey Paul,
This is up your alley.
I had a similar situation... but I had a couple walls to support most of the CJs across the center. There were a few that I doubled up the CJs on each side and hung a double 2x header between, then hung the CJs on joist hangers attached to those headers. Inspector had no issues with it.
Thats pretty much what I will end up doing, I was jsut looking for the formula for calculating what size wood would be needed.
I could probably enroll in archy101 and I'd have my answer in the first week.
Whatever you ceiling joists are, just double them just like a stairwell opening.
Lets see if I can talk this out and reasonably deduct an answer.(I do this outloud on the jobsite at times)
I have a cj(A) with a span of 12'6". That cj will have to carry 1/2 the span(lets say 6.5ft) of 9 cj that run perpendicular to A.
So A not only has to carry its own weight 12'6" but also another 70 sq ft of ceiling load.
So if a ceiling dead load is 5 psf, would A have to be able to carry and an additional 350lbs over 12'6"?
Would that then be considered 28 psf + the original 5 to make it 33 psf?
Is this how it is done?
I punched in the #'s and just changed it from cj's(live load 20, dead load 10) to floor joists with a live load of 40 and a dead load of 20(max it would allow) and it spit out 1 2x8 would be sufficient. (deflection l/360)
Joe, it sound like your "just double it like a stairwell" comment is pretty close to right on. Do you have a formula to calculate this? What if I had to hang 20' cj from A?
There is a tremendous amt of info missing in this whole thing, such as how the ridge is supported and what it is made of. Where supporting walls intersect, and how the shear resists racking in the outer walls. Also, when you say ridge at 16' and cieling joists at 9', is that above floor or above top plate?This could be a simple thing like Joe suggests or it could be an immensly complicated calculation that needs some analysis from an engineer.That said, I think from what I understand, that you should be able to do a doubled 2x121 for the supporting CJ and then 2x5s for all the rest
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I understand that more info may be needed. For ease of calculations I tried to generalize.let's try this.The roof(framed off the top of 2flr floor joist like a bungalow) is designed to support cathedral ceilings throughout. Ridge, valleys and posting are sized accordingly .The prints give no layout for the "optional"(blueprint wording) cjs.Most of the ceiling will be cathedral except thisone area.I was just looking for the calcs to use to figure the sizing for the cj that will be used to hang the others off of.The one Area that could be of concern would be where the dbl cj will be nailed to the valley and if the valley could support the extra weight at that spot. With the minimal extra weight I don't think that will be a problem though.Fwiw the valleys are 2 - 28' - 11 7/8" x 1 3/4" microlam and the main ridge is 4 42' - 18"x 1 3/4"micro. Rafters are 2x12
Well.. first figure the square feet of ceiling that will be "hung" from the doubled up CJs. 12'6" x whatever the distance between the doubled up CJs. Multiply that by the load it needs to support, basically just drywall and insulation above (usually 15 psf or so). Divide by 2. That is what each doubled up CJ is carrying. Divide that by your length of doubled up CJ (12'6") and you get pounds per lineal foot (PLF). Find a span table (I don't have any with me) and find the acceptable framing member to support that load. Doubled up 2x something will probably work, or an lvl if you would prefer that. You can get skewed joist hangers to hang the doubled up 2x or lvl from the valley rafters. The point loading on the valley is a problem for an engineer... but I wouldn't lose any sleep over it.
Edited 12/30/2008 1:25 pm ET by Doobz26
Here is an online calculator for beam sizing.
http://www.forestryforum.com/members/donp/beamclcNDS2.htm
You'll need to look up some physical properties of the species of wood you are using. Not sure if i did it right or not but it looks like a doubled up 2x8 beam "passes."